The Stephen R. Donaldson Universe Wiki

Ranyhyn are the great horses of the Land. These horses live on the Plains of Ra, and are tended by the Ramen. The Ranyhyn are akin to normal horses, but are larger, always have a star and are in some indefinable sense enhanced by the Earthpower of the Land, so that their speed and endurance, as well as their intelligence, far outstrip those of a standard horse. The Ranyhyn can be ridden by individuals they deem worthy, but a person who seeks such a mount must travel to the Plains of Ra and offer himself to the horses for consideration. If a Ranyhyn accepts a rider, it is loyal to that rider until death. All of the Bloodguard (apparently) are accepted by the Ranyhyn, but not all Lords have been deemed worthy. The Ranyhyn also have a limited ability to perceive the future; these horses can "hear" when their rider will need them, hearing their calling days or weeks before the rider makes the call. Thus, when the rider summons his Ranyhyn, it appears shortly thereafter, regardless of the distance between them.